Repaired shroud assembly for a boiling water reactor

ABSTRACT

A method of repairing nuclear reactor core shrouds having horizontal cracks in heat affected zones of welds includes applying vertical compressive forces to the shroud in situ to urge opposed surfaces of the cracks toward one another. A plurality of tie rods, angularly spaced about the shroud periphery, is used to selectively apply the longitudinally compressive forces. Preferably, the method utilizes existing hardware secured to the shroud for attaching the tie rods thereto. The tie rods may be used in communication with horizontal and/or vertical spacers inserted between the shroud and the reactor vessel. Horizontal spacers serve to resist horizontal seismic loads and hold the shroud in place in spite of cracking. Vertical spacers resist vertically upward loads resulting from main steam line breaks.

This application is a continuation of patent application Ser. No.08/334,361, filed Nov. 3, 1994, now abandoned, which is a continuationapplication of patent application Ser. No. 08/190,796, filed Feb. 2,1994, now U.S. Pat. No. 5,402,570.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to methods for repairing nuclear reactorcore shrouds. In particular, the method of the present invention isconcerned with repair of boiling water reactor shrouds in order to allowsafe reactor operation even with extensive cracking along heat affectedzones of horizontal welds.

2. Discussion of the Prior Art

Boiling water reactor shrouds are disposed concentrically within thereactor vessel and are commonly formed from multiple arcuate steelplates having a thickness on the order of one and one-half inches.Plates at each horizontal level of the shroud are joined by welds alongabutting, relatively short, vertical edges to encircle the reactor atthat level. The plates at different adjacent levels are joined alongtheir abutting, relatively long, arcuate edges with horizontal welds.After periods of use, cracking of the shroud tends to occur within heataffected zones of the welds as a result of corrosion, radiation andstress. Cracking of the vertically oriented welds is consideredacceptable because these welds are relatively short in length, relativeto the overall shroud length, and do not adversely affect the functionof the shroud (i.e., support and alignment of the nuclear fuelassemblies, and channeling of reactor coolant flow). Specifically,vertical welds at adjacent levels are offset angularly about the shroudperiphery so that cracking of such a weld crank extend, at most, onlythe axial or vertical length of that level. However, if cracking occursalong the longer horizontal or circumferential welds, a crack can extendalong the entire circumference or periphery of the shroud, permittingrelative lateral movement between the plate levels. Such excessivecracking, therefore, could prevent the core from supporting and aligningthe fuel assemblies, can improperly direct or impede coolant flow, andmay permit coolant flow leakage.

When excessive horizontal weld cracking occurs, the shroud must eitherbe replaced or repaired. Repair is certainly the preferred alternativein view of the fact that replacement involves significant expense,relatively long shut down time, and the potential for radiation exposureto personnel. To date, however, there has been no acceptable method ofshroud repair in situ. Repair techniques used to date typically involvebolting brackets onto vertically adjacent plates across a weld crack.This approach requires plural brackets for each crack, depending uponthe length of the crack. Moreover, welds must be separately inspectedafter repair for additional cracking of the repaired welds as well asfor new cracks in other welds.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor repairing in situ nuclear reactor shrouds of the type havingmultiple levels of arcuate plates welded together, the method permittingsafe reactor operation even with excessive cracking of horizontal welds.

It is also an object of the present invention to provide such method inan already operational reactor where the repair method is relativelyinexpensive, reactor shut-down time is minimal during the repairprocedure, and radiation exposure is negligible for repair personnel.

It is also an object of the present invention to provide a method forrepairing previously operational reactor shrouds having cracks alongheat affected zones of horizontal welds, said method being accomplishedwithout using structures that significantly impedes coolant flow in theannular space between the shroud and the vessel.

A further object of the present invention is to provide a method forrepairing a horizontally cracked reactor shroud in a manner thatsubstantially diminishes the need for subsequent inspection for weldcracks.

In accordance with the present invention, a method for repairing analready operational reactor shroud of the type described involvesinspecting the shroud for horizontal cracking along welds and securingplural tie rods in vertical orientation spaced about the periphery ofthe cracked shroud to axially compress the shroud and thereby urge theopposing surfaces of a horizontal crack toward one another.Alternatively or in addition, spacers may be compressibly installedbetween the outer wall of the shroud and the inner wall of the reactorvessel at different longitudinally spaced and angularly spacedlocations. In a preferred embodiment the tie rods and spacers are bothinstalled to provide maximum protection against relative transversemovement between the shroud levels along horizontal cracks.

The method of the present invention repairs existing horizontal weldcracks and permits continued operation in spite of subsequentlyoccurring horizontal weld cracks by applying the axially compressiveforce on the shroud assembly with the tie rods, and by continuouslyproviding radially exerted stabilizing forces with the spacers.

The foregoing and other objects, features and many of the attendantadvantages of the present invention will be appreciated more readily asthey become better understood from reading the following descriptionconsidered in connection with the accompanying drawings wherein likeparts in each of the several figures are identified by the samereference characters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut away view in perspective of a boiling water reactorexemplifying a type of reactor suitable for repair by the method of thepresent invention.

FIG. 2 is a flattened projection of a cylindrical core shroud in thereactor of FIG. 1.

FIG. 3 is a detailed view in broken longitudinal section of a portion ofa reactor shroud repaired according to the present invention.

FIG. 4 is a view in section taken along lines 4--4 of FIG. 3.

FIG. 5 is a view in section taken along lines 5--5 of FIG. 3.

FIG. 6 is a top view in plan of a tie rod assembly employed in oneembodiment of the method of the present invention.

FIG. 7 is a front view in broken elevation of the tie rod assembly ofFIG. 6.

FIG. 8 is a detailed view in longitudinal section of a portion of areactor shroud repaired using the tie rod assembly of FIG. 6.

FIG. 9 is a detailed view in broken longitudinal section of portion of areactor shroud repaired in accordance with another embodiment of themethod of the present invention.

FIG. 10 is a view in section taken along lines 10--10 of FIG. 9.

FIG. 11 is a view in section taken along lines 11--11 of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to provide a point of reference for understanding the repairmethod of the present invention, a boiling water nuclear reactorassembly 10 is illustrated in cut-away in FIG. 1. In view of the factthat the invention relates to repairing the reactor shroud and not tothe operation of the reactor itself, no attempt is made herein toprovide a detailed description of reactor operation. Such operation isadequately described in numerous publications and patents, for exampleU.S. Pat. Nos. 3,627,634 (Guenther et al) and 4,789,520 (Morimoto etal), among others.

Reactor assembly 10 includes a pressure vessel 11 tightly sealed by alid 12 at the vessel top. Reactor core shroud 13 is mounted insidevessel 11. A steam separator 14 is mounted on a steam plenum head 20 ontop of the shroud 13, and a steam drying assembly 15 is disposed abovethe steam separator. A lower reactor core support plate 16 disposedwithin shroud 13 supports a fuel assembly 17, also disposed within theshroud. Lower core grid 18 and upper core grid 19 are positioned belowand above, respectively fuel assembly 17.

Control rod guide tubes 21 are provided within vessel 11 at locationsabove a control rod driving mechanism (not shown) located at the bottomof the vessel beneath shroud 13. The lower ends of corresponding controlrods 23 are detachably connected to the driving mechanism and arearranged to move up and down within guide tubes 21.

Recirculated water is delivered into vessel 11 via one or more inletports 24, and egresses via one or more outlet ports 25. Also illustratedis a diffuser 26, a core sparger 27 and a feed water sparger 28. One ormore steam outlets 29 communicates between the interior and exterior ofvessel 11 at a location above the shroud and the steam separator.

Shroud 13 is typically made up of multiple levels, each comprisingplural arcuate Type 304 steel plates having a thickness on the order ofone and one-half inches and welded together at their abutting edges. Tobetter understand this welded structure, reference is now made to FIG. 2wherein the plates are shown in flattened projection adjacent a 360°scale provided for angular orientation reference. The plates areoriented with their long or arcuate dimensions disposed horizontally,their width dimensions disposed vertically and their thicknessdimensions disposed radially or into the plane of the drawing in FIG. 2.

The particular shroud illustrated in FIG. 2 includes eight horizontalannular levels of plates, the plates at any level being of the samewidth (or height), with the height varying from level to level. Thethickness or depth of the plates is the same for every level. Asillustrated, the bottom annular level is made up of two plates 30, 31,each 180° in circumferential length, disposed end to end and weldedtogether at their abutting ends to form vertical welds 32 and 33. Thesecond level from the bottom is made up of six plates, 34, 35, 36, 37,38, 39, each 60° in circumferential length, also welded together atabutting ends to form vertical welds 40, 41, 42, 43, 44 and 45. Thebottom edges of plates 34 through 39 are welded to the abutting topedges of plates 30, 31 to provide a circumferentially continuoushorizontal weld 46. The vertical welds 32 and 33 between plates 30 and31 are angularly offset from vertical welds 40 through 45 so thatvertical welds in adjacent levels are not in angular or longitudinalalignment. It is also noted that the plates 34 through 39 in the secondlevel are significantly narrower (i.e., smaller in vertical height) thanplates 30, 31 in the bottom level.

The third lowest level includes two 180° plates 50, 51 disposed end toend and welded together at their abutting ends to form vertical welds52, 53. The bottom edges of plates 50, 51 are welded to the top edges ofplates 34 through 39 to provide a circumferentially continuoushorizontal weld 54. Vertical welds 52, 53 are angularly offset fromvertical welds 40 through 45 and from welds 32, 33. Plates 50, 51 arewider (i.e., larger in the vertical dimension) than plates 30, 31 andplates 34 through 40.

The fourth annular level of shroud plates includes two 180° plates 55,56 having their ends welded together at welds 57, 58. Vertical welds 57,58 are angularly offset from vertical welds 52, 53 and from welds 40through 45; however, welds 57, 58 are vertically aligned with welds 32,33, respectively. Plates 57, 58 are wider than the plates in all otherlevels of the shroud. The bottom edges of plates 55, 56 are welded tothe top edges of plates 50, 51 to form a circumferentially continuoushorizontal weld 59.

In a like manner, successively higher annular levels of the shroudincludes two 180° plates 60, 61 at level five, six 60° plates 64, 65,66, 67, 68, 69 at level six, two 180° plates 80, 81 at level seven, andsix 60° plates 84, 85, 86, 87, 88, 89 at level eight. Level five hasvertical welds 62, 63; level six has vertical welds 70, 71, 72, 73, 74and 75; level seven has vertical welds 82, 83; and level eight hasvertical welds 90, 91, 92, 93, 94, 95. Circumferentially continuoushorizontal welds 76, 77, 78 and 79 are provided between levels four andfive, levels five and six, levels six and seven and levels seven andeight, respectively. The widths or heights of level five, six, seven andeight are all different, each being smaller than the widths of levelsthree and four.

No vertical welds in any shroud level are aligned with vertical welds inan adjacent shroud level. Accordingly, a crack in a heat affected zoneof any vertical weld cannot extend beyond one shroud level, and suchcracks are typically ignored. On the other hand, horizontal welds 46,54, 59, 76, 77, 78 and 79 are circumferentially continuous. A crack inthe heat affected zone in one of these horizontal welds, therefore,could ultimately extend about the entire circumference of the shroud. Itwill be appreciated that circumferential cracks could result insignificant relative horizontal movement between shroud levels inresponse to seismic events and other vibrations. This would adverselyaffect the support and alignment of nuclear fuel assemblies disposedwithin the shroud. Likewise, coolant flow through the reactor would beadversely affected.

The repair method of the present invention is performed upon the firstindication of cracking in the heat affected zones of horizontal welds ina reactor core shroud of the type described. Specifically, if horizontalcracking is noticed during a periodic inspection of the shroud, theshroud can be repaired in the manner described below. The repair methodinvolves installing tie rods and/or horizontal spacers and/or verticalspacers on the shroud in situ while the reaction is shut down.Installation of the tie rods uses existing structure in the reactorwhenever possible, but may require additional hardware. The tie rods arespaced about the shroud periphery in the annular space between theshroud and the reactor vessel. A particular shroud structure is shown inFIGS. 3, 4 and 5 to illustrate an exemplar tie rod repair installation.

Referring to FIGS. 3, 4 and 5, shroud 113 has a frusto-conical supportstructure 114 diverging from its bottom to abut the interior surface ofreactor vessel 111. Support structure 114 is welded to the vessel wallat the annular abutment location. The upper edge of the supportstructure may be formed integrally with or welded to the bottom annularedge of shroud 113. A plurality of brackets 115 are secured by weldingor the like to the outside of the shroud cylinder near its bottom, andto support structure 114 to provide additional strength for the junctionof the support structure and the shroud. Brackets 115 are spaced atregular angular intervals about the periphery of shroud 113. As shown,the lowermost part of bracket 115, extending along the support structure114, is radially spaced from the wall of vessel 111. The shroud sealinglid 112 is secured to the top of shroud 113 by means of bolts 116threadedly or otherwise engaged by lugs 117 secured in angularly spacedrelation to the shroud periphery adjacent the upper edge of the shroud.It is to be noted that, for purposes of convenience, the arcuate platesmaking up the different shroud levels are not individually illustratedin these drawings; it is to be understood, however, that the shroudstructure comprises a plurality of horizontal levels of plates of thetype described above in relation to FIG. 2. The structure thus fardescribed is part of the original reactor assembly and is not added aspart of the repair method.

The tie rods 120 employed to repair the shroud are engaged by using theexisting brackets 115 and lugs 117. Specifically, the bottom of each tierod 120 has a hook or similar engagement member 121 secured at itsbottom end. Engagement member 121 is configured to permit a portion 122thereof to engage a bracket 115 from the underside of that bracket toprevent upward axial movement of the tie rod. It will be appreciatedthat, for reactors wherein brackets 115 are not provided, suitable holesmay be formed in support structure 114 to receive and threadedly (orotherwise) engage the bottoms of tie rods 120. Under such circumstances,of course, engagement member 121 is not reinforced. Holes for thispurpose may be formed by drilling, EDM techniques, etc.

At their upper ends, tie rods 120 are secured to respective metal beams123 installed during the repair procedure atop the existing lugs 117. Inparticular, each beam 123 is supported between a respective pair ofangularly spaced lugs 117 and is secured to the lugs by any suitablemeans, such as by adhesive, screws, etc. A hole is defined verticallythrough each beam 123 and is configured to receive a respective tie rod120 extending therethrough. Each tie rod 120 may be axially tightened bya respective nut 124 threadedly engaging its upper end and bearingagainst beam 123. When nuts 124 are tightened, hooks 121 are pulledupward such that portions 122 exert upward forces against brackets 115.Corresponding downward forces are exerted by the nuts on the beams 123,resulting in axial or longitudinal compression of the entire shroud bythe multiple, angularly spaced tie rods. This longitudinal compressionurges the opposing surfaces of horizontal cracks toward one another,thereby sealing the cracks and preventing their adverse effects on theshroud structure. Moreover, horizontal cracking occurring subsequent tothe repair procedure is rendered similarly ineffectual by the axialcompression continuously applied by the tie rods.

An alternative tie rod assembly and mounting arrangement is illustratedin FIGS. 6, 7 and 8 to which reference is now made. The engagementmember 131 secured to the bottom of the tie rod assembly has its bottomportion 132 formed with a radially outward facing surface 133 contouredand arranged to abut the interior wall of reactor vessel 111 in flushrelation when the engagement member is deployed in engagement withbracket 115. Also, the surface of portion 132 that engages bracket 115is positioned at an angle close to perpendicular to shroud support 114during deployment. Accordingly, when the tie rod is tightened, a lateralforce component is applied to member 131 to urge its surface 133radially outward against the wall of vessel 111. The result is lateralsupport for the shroud in addition to the longitudinal compressionapplied by the tie rods.

Another difference in the tie rod assembly of FIGS. 6, 7 and 8 is thepresence of an outer tube 135 disposed concentrically about tie rod 120with a small radial clearance. The bottom of tube 135 is fixedly securedto the top of engagement member 131. The upper end of tube 135terminates somewhat below the top of tie rod 120 so as to be spacedsomewhat below beam 123 when the tie rod is deployed. A spacer 136 takesthe form of a tubular projection extending radially from tube 135 in thesame angular direction as surface 133 of engagement member 131. Spacer136 is located near the upper end of tube 135. The length of spacer 136is selected such that it bears axially against the interior surface ofvessel 111 when the tie rod is deployed.

Deployment of the tie rod assembly of FIGS. 6, 7 and 8 involves loweringthe assembly vertically downward between the shroud and vessel in anorientation where it is rotated 90° relative to its final deploymentorientation. This permits engagement member 131 and spacer 136 to befreely moved longitudinally until the assembly is at its proper verticalposition. The assembly is then rotated 90° about the tie rod axiswhereupon engagement member 131 engages bracket 115, and surface 133 andspacer 136 bear radially against the vessel wall.

Although beams 123 are a convenient structure for securing tie rods 120to lugs 117, other means may be employed for the stated purpose. The tierods may be axially preloaded during the repair procedure, all left withsome allowance for differential expansion during reactor operation.

The tie rods themselves may be open-ended tubes, rather than solid rodsas shown, in order to resist lateral vibration while providing minimumresistance to reactor coolant flow between the shroud and vessel, andproviding minimum displacement of water for radiation shieldingpurposes.

The tie rods serve two primary functions. First, they axially compressthe horizontal shroud levels together, even for a worst case horizontalcrack extending a full 360° about the shroud. The design basis verticalload capability of the tie rods for this purpose is on the order of 1.5to 2.0 million pounds, depending upon the actual reactor being repaired.Second, the tie rods prevent lateral shear deflection between shroudlevels which could otherwise be free to move laterally due to a 360°circumferential crack fully through the shroud wall thickness. Theprevention of lateral shear is effected by compressing the oppositesurfaces of the horizontal cracks against one another. The design basislateral load due to seismic events is approximately 400,000 pounds,depending upon the reactor.

In the embodiment illustrated in FIGS. 9, 10 and 11, lateral spacers areinstalled between the inside wall of reactor vessel 111 and the outsidewall of shroud 113. Each tie rod 120 has an upper spacer 140 and a lowerspacer 150 associated therewith. These spacers serve to resisthorizontal seismic loads and hold the shroud in place relative to thevessel wall in the event of cracking along any horizontal weld during aseismic event. Upper spacers 140 are plate-like members disposed inrespective radial planes about the shroud axis with their oppositelongitudinal edges abutting the shroud and vessel. Each spacer 140 isdisposed between a respective pair of lugs 117. Spacer 140 has a pair ofangled braces 141, 142 secured to respective spacer surfaces andextending to the junction between the facing lug 117 and shroud 113.Braces 141, 142 may be welded or otherwise secured to spacer 140 andprovide stable three-site contact with the shroud surface to resistrotation of the spacer plate out of its radial plane. Each spacer 140includes a bore extending therethrough from its top edge to its bottomedge to receive and possibly engage tie rods 120. The spacer bore may bethreaded to serve as a tightening nut that bears against a protrusion143 from the upper end of the shroud. Alternatively, the bore may besmooth, requiring a separate tightening nut to apply the axiallycompressive forces.

Spacer 150 has a bottom portion adapted to be secured to the top of arespective bracket 115 or other engagement structure near the bottom ofshroud 113. Engagement may be by means of a hooklike structure or anyother suitable means. During insertion of the tie rod assembly, theassembly is rotated such that spacer 150 is perpendicular to its finaldeployment position. When disposed at the desired vertical location, theassembly is rotated by 90° to cause spacer 150 to be wedged between theshroud and the vessel.

In addition to the horizontal spacers 140, 150, additional spacers maybe installed to resist upwardly directed loads. Specifically, suchspacers would rest on respective lugs 117 and have upper ends abuttingthe bottom of feedwater spargers 160 disposed at spaced angularlocations above lid 112. Such vertical load spacers would include a beamstructure, similar to beam 123, resting atop lugs 117 that are not usedto support tie rods 120. The upwardly directed vertical loads resistedby these spacers would be those due to a main stream line breakaccident. The vertical load spacers hold the shroud down and in placeshould there be a horizontal crack.

Horizontal and vertical spacers may be installed in any combination andmay be utilized with or without tie rods. If tie rods are used inconjunction with horizontal spacers, for example as illustrated in FIGS.9, 10 and 11, the number of tie rods required to effect the necessaryaxial loading and render horizontal cracking ineffectual is reduced byapproximately half.

Although a variety of different types of attachment of the lower end ofthe tie rods has been described and illustrated, it should be understoodthat the method of such attachment is not to be limiting. For example,the lower end of the tie rod can hook onto an existing bracket, beattached to a gusset, or utilize a wedge-type spreading device thatautomatically wedges itself between the shroud and vessel at the bottomof the shroud, etc.

The repair method described herein utilizes the addition of tie rodsand/or horizontal and/or vertical spacers installed in situ when thereactor is shut down. Approximately twenty tie rods are required for atypical installation. Alternatively, or in combination, approximatelyten sets of horizontal and/or vertical spacers are utilized in the giveninstallation.

As noted, in most instances the tie rods are secured to existinghardware, although hardware may be added, or other suitable means forengagement may be provided for the tie rods.

The repair method of the present invention has numerous advantageousfeatures. For example, the method accommodates any degree of expectedcracking within the stainless steel portion of the shroud, including itssupport ring. The method is more than adequate for all existing designbasis loads including normal loads such as vibration, andaccident-related loads such as steam and recirculation line breaks,seismic events, etc. The repair method serves as a permanent repair ofthe shroud once horizontal cracks are noticed. In addition, the repairinstallation substantially reduces the need and extent for any futureinspections for shroud cracking. The repair method is effected withoutmodification of existing reactor internal equipment and withoutrefueling procedures for most reactors; in some cases some minoradditional hardware may be required. Additionally, the method may beperformed without any welding required inside the reactor vessel.

From the foregoing description it will be appreciated that the inventionmakes available a novel method for repairing boiling water reactorshrouds so as to completely overcome any deleterious effects ofhorizontal of the shroud.

Having described preferred embodiments of a new and improved method forrepairing boiling water reactor shrouds in accordance with the presentinvention, it is believed that other modifications, variations andchanges will be suggested to persons skilled in the art in view of theteachings set forth herein. It is therefore to be understood that allsuch variations, modifications and changes are believed to fall withinthe scope of the present invention as defined by the appended claims.

What is claimed is:
 1. A repaired shroud assembly for a nuclear boilingwater reactor comprisinga shroud having a substantially cylindricalconfiguration formed of multiple levels of arcuate plates joined bycircumferentially continuous horizontal welds, said shroud having topand bottom structures; and a plurality of tie rods connected betweensaid top and bottom structures at a respective plurality of angularlyspaced locations about an exterior of said shroud, each of said tie rodsbeing connected with said top and bottom structures of said shroud intension to repair said shroud by compressing said plate levels alongsaid horizontal welds to prevent relative lateral movement of said platelevels adjacent cracks along said horizontal welds.
 2. A repaired shroudassembly for a nuclear boiling water reactor as recited in claim 1wherein an upper end of each tie rod is secured to a beam depending fromsaid top structure of said shroud.
 3. A repaired shroud assembly for anuclear boiling water reactor as recited in claim 2 wherein said topstructure of said shroud includes a plurality of angularly spaced lugsand said beams are mounted between pairs of said lugs.
 4. A repairedshroud assembly for a nuclear boiling water reactor comprisinga shroudhaving a substantially cylindrical configuration formed of a pluralityof plates joined by horizontal welds, said shroud having top and bottomstructures; and a plurality of tie rods disposed around a periphery ofsaid shroud and connected between said top and bottom structures, eachof said tie rods being connected with said top and bottom structures ofsaid shroud in tension to compress said plates along said horizontalwelds whereby said tie rods prevent radial displacement of said platesadjacent cracks along said horizontal welds; wherein said bottomstructure of said shroud includes a plurality of angularly spacedbrackets and a lower end of each tie rod is secured to a hook configuredto engage an underside of one of said brackets.
 5. A repaired shroudassembly for a nuclear boiling water reactor as recited in claim 1wherein said shroud is disposed within a reactor vessel and furthercomprising a plurality of radial spacers mounted on said tie rods andengaging a wall of said reactor vessel.
 6. A repaired shroud assemblyfor a nuclear boiling water reactor as recited in claim 1 wherein saidshroud is disposed within a reactor vessel and further comprising aplurality of radial spacers mounted on said tie rods and extendingbetween said shroud and a wall of said reactor vessel.